Combine basil, cilantro, parsley, and thyme in a bowl. Reserve 2 tablespoons in a bowl and refrigerate. Place 1/2 of the herb mixture in the bottom of a large soup pot. Add 1/2 the tomatoes, 1/2 the bell peppers, and the potatoes in layers. Top with lamb and season with cumin, mint, salt, and black pepper. Add eggplant, carrots, remaining tomatoes, remaining bell peppers, and remaining herb mixture. Pour in beer.
Cover the pot with a lid and cook over medium heat about 1 hour. Add onions and garlic. Season with salt if necessary and cook 2 hours more. Garnish with reserved 2 tablespoons herb mixture and serve hot.